I would call sprint and let them know it was found. Since you already had it replaced when you thought it was lost and gone forever, they're not going to tell you return the replacement and switch back to the old phone. Depending on your point of view, the worse thing they'll want you to turn in the old phone. Personally, I think that would be a nice option because what else are you going to do with it? To keep it means you have a WiFi only 5.5" tablet, or you have to open a new line to use it as a phablet. And I find having anything less than a 10" tablet is a waste when I have the GN2.
I just see a whole lot of problems if you're discovered with the phone and it's still believe to be stolen. Not that I think Sprint would go after you, but there is very slim possibly they could go after you for fraud.